1. Field of the Invention
The invention relates to an image intensifier tube comprising an entrance screen with a photocathode and an exit screen with a phosphor layer for converting photoelectrons from the photocathode into radiation, which exit screen comprises an image detection matrix for deriving an electronic signal from the radiation.
2. Description of the Related Art
An image intensifier tube of this kind is known from NL 90 00 267 (PHN 13.212) laid open to public inspection.
The known image intensifier tube is an X-ray image intensifier tube in which an image-carrying X-ray beam is converted into light which is incident on the photocathode so as to generate an image-carrying electron beam. A vacuum exists in the gastight envelope in order to minimize electron beam scattering. The image-carrying electron beam is imaged on the phosphor layer by the electron-optical system. The electron beam incident on the phosphor layer generates light therein, which light is emitted by the photocathode. The light generated in the phosphor layer is emitted in the forward direction towards the image detection matrix in which the image information is converted into an electronic image signal. If no steps are taken, the phosphor layer also emits light in the backward direction, i.e. towards the photocathode. The light emitted in the backward direction by the phosphor layer and incident on the photocathode generates an additional, disturbing electron beam. The phosphor layer also converts the additional, disturbing electron beam into light which is detected by the image detection matrix and thus disturbs the electronic image signal.
U.S. Pat. No. 4,140,900, issued Feb. 20, 1979, discloses a reflecting aluminium layer provided on the phosphor layer of an X-ray image intensifier tube. Such a reflecting layer ensures that light emitted backwards by the phosphor layer cannot reach the photocathode. Providing an aluminium reflecting layer on the phosphor layer when the latter has been provided on a semiconductor image detection matrix, however, is not possible by means of known technology. The deposition of such an aluminium reflecting layer requires a process step during which the phosphor layer with the image detection matrix is exposed to a high temperature, notably higher than 400.degree. C. When a semiconductor image detection matrix is exposed to a temperature higher than 200.degree. C., the operation of the image detection matrix will be affected.